Harmonic function, Engineering Mathematics

Let z0 = a + ic, z1 = b + id, z2 = -id, and z = [z0 + z1]. Let z0 be the apex of the wedge with one ray passing through z1 and the other passing through z2, and with z in its interior. Suppose φ is a harmonic function in the wedge that contains z, z1, z2, and z0, taking the value -0.5 on the ray passing through z1 and the value 1.5 on the ray passing through z2. What, to 3 decimal places, is the value of φ at z?

(Hint: if you can't figure out how to get the right quadrants for your angles, try breaking them into pieces - referencing each piece separately to the nearest axis, +y or -y or -x - so that each piece is between 0 and 90 degrees; then sum them up.)